fix: correct demo navigation and touch backdrop dismissal

[rosek86]

Fix two demo navigation issues and refresh the README preview.

• Use explicit api/index.html links throughout the examples and local development instructions. Vite serves /api/ through its demo fallback, whereas GitHub Pages resolves the directory index. Keep deep links and standalone gallery links intact.
• Replace the desktop settings backdrop with a native button excluded from sequential keyboard focus. Use native button activation for mouse, touch, and keyboard; retain the outside-press guard for the mobile modal backdrop, chart interaction, Escape, and focus restoration. The existing WebKit touch test remains unchanged.
• Replace the README screenshot with the current graphite dark theme, matching panel heights, one trace, a marker, and cursor readings.

Validation:

• Confirmed local documentation navigation from the demo, gallery, React, and basic example, including API search.
• npm run lint:fix, npm run format, and npm run check passed.
• Full Chromium and WebKit suites passed locally (125 tests each).
• Visually reviewed the new README screenshot.
• Final Linux CI passed, including Chromium, WebKit, Firefox, package-consumer checks, and standalone examples: https://github.com/rosek86/smithkit/actions/runs/38507727789